The cell and developmental biology of the human pathogen, Leishmania, and related trypanosomatid parasites are investigated as models of both intra and extracellular parasitism. Emphasis is placed on characterizing the basic functions and gene structure of unique parasite surface membrane enzymes and related proteins toward defining their roles in parasite survival and development. A probe from the L. donovani(Ld)3'-nucleotidase/nuclease (e'-NT/Nu) gene was used to map the locus of this gene to a single about 2.5 Mb sized Ld chromosome which was also present in a variety of other pathogenic Leishmania species. Thus, the basic structure of this gene has been conserved across species lines. The nutrient-induced over-expression of Ld 3'-NT/Nu enzyme activity ws shown to be regulated by both transcriptionally mediated events and posttranslational modification of the newly synthesized protein. In homologous episomal transfection-expression studies, the Ld surface membrane 3'NT/Nu was converted into, and expressed as, a soluble, enzymatically active secretory protein by deleting the transmembrane anchor domain from its gene. In other studies, the gene for the unique Ld surface membrane acid phosphatase (MbAcP) was identified and characterized. An MbAcP gene-probe was used to map the locus of this gene to a single about 1.45 Mb sized Ld chromosome which was also present in various different pathogenic Leishmania species indicating that the basic structure of this gene has also been conserved across species boundaries. We also recently identified a new 38 kDa surface membrane nuclease present in both Ld pro and amastigotes. Based on its surface localization and constitutive expression, we assume that this enzyme must play an essential role in parasite acquisition of host-derived nucleo-bases. Finally, the gene encoding the leishmanial homolog of calreticulin was characterized. Its expressed protein possessed multiple functions including both high Ca++ binding and autokinase activities and, most significantly, phosphorylation-independent binding activity for homologous leishmanial RNAs. The latter suggests that calreticulin may play a role in RNA processing, transport or protection in these parasites.